Aluminum oxide (alumina, Al2O3) exists in many metastable polymorphs such as xcex3, xcex7, xcex8, xcex4, xcexa, and "khgr" in addition to the thermodynamically stable xcex1-Al2O3 phase (corundum). When produced by chemical vapor deposition (CVD), Al2O3 crystallizes into xcexa-Al2O3 and xcex8-Al2O3 modifications in addition to the stable xcex1-Al2O3. Earlier CVD coatings were usually mixtures of several polymorphs, the most commonly occurring metastable Al2O3 modification being xcexa-Al2O3. Today, both xcex1-Al2O3 and xcexa-Al2O3 polymorphs are used as hard coatings, and they can be deposited in a controlled way by modern CVD technology shown, for example in U.S. Pat. Nos. 5,137,774 and 5,700,569.
Dopants of hydrogen sulfide (H2S), phorphorus chloride (PCl3) carbonoxysulfide (COS) or phosphine (PH3) can be applied in order to increase the growth rate and thickness uniformity of the alumina layers. The most commonly applied dopant is H2S as also disclosed in U.S. Pat. No. 4,619,886. While amounts of H2S in the total CVD gaseous mixture of from 0.003 to 1% by volume and temperatures of 700 to 1200xc2x0 C. are broadly disclosed in that patent, all exemplifications of the process therein are below 0.5 vol % and generally around 0.1 to 0.3 vol % H2S used at temperatures of 1000-1030xc2x0 C. H2S has been called the xe2x80x9cmagical dopantxe2x80x9d in view of its effect on improving the growth rate and uniformity of Al2O3 coatings applied by conventional CVD techniques at temperatures around 980xc2x0 C. See, Oshika et al., xe2x80x9cUnveiling the Magic of H2S on the CVD-Al2O3 Coatingxe2x80x9d, J. Phys IV France 9 (1999), Pr 8-877-Pr 8-883.
CVD xcexa-Al2O3 is considered to exhibit morphological advantages (smaller grain size and lower porosity), lower thermal conductivity and even a higher hardness when compared with the CVD xcex1-Al2O3 phase. These are important properties when metal cutting is concerned. However, at the relatively high temperatures ( greater than 1000xc2x0 C.) reached during metal cutting, metastable xcexa-Al2O3 may transform to the stable xcex1-Al2O3 polymorph. xcex3-Al2O3, when deposited using physical vapor deposition (PVD) or plasma assisted CVD has been found to exhibit high hardness and good wear properties. See, for example, WO 9924634 and U.S. Pat. No. 5,879,823. However, xcex3-Al2O3 has not been available using conventional CVD techniques.
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide xcex3-Al2O3 using conventional CVD.
In one aspect of the invention there is provided a coated body having as the outer layer, a layer of xcex3-Al2O3 deposited by chemical vapor deposition.
In another aspect of the invention there is provided a method of forming a coated body having a layer of xcex3-Al2O3 comprising coating the body with a gaseous mixture of AlCl3, CO2, H2 and H2S at a temperature of from about 600 to 800xc2x0 C., the H2S being present in amounts of at least 0.7% of the total mixture.